Test means, test device and method for determining the ionic strength or specific gravity of a liquid sample

ABSTRACT

A test means for determining the ionic strength or specific gravity of an aqueous test sample, the test means comprising a weakly acidic or weakly basic polyelectrolyte which is at least partially neutralized, and an indicator means capable of producing a detectable response to ion exchange between the polyelectrolyte and the sample. The test device comprises a carrier matrix incorporated with the test means, and the method for its use comprises contacting an aqueous test sample with the device and observing a detectable response.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a Continuation-in-part of United StatesPatent Application Ser. No. 958,630, filed Nov. 8, 1979, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the determination of the ionic strengthor specific gravity of a test sample. More particularly it relates to atest means, test device and method for determining the ionic strength orspecific gravity of an aqueous test sample.

2. Description of the Prior Art

The determination of the specific gravity of a liquid has application innumerous arts. Thus, such unrelated disciplines as brewing, urinalysis,water purification, preparation of drinking water aboard a ship at sea,etc., all involve the measurement of specific gravity. Needless to say,a quick, facile method for determining this property would greatlyenhance the state of many scientific arts, including any technologywhere rapid, accurate determination of specific gravity would bebeneficial. Thus, for example, if a medical laboratory technician couldaccurately measure the specific gravity of a urine sample in a matter ofseconds, not only would the rapid results aid the physician indiagnosis, but also laboratory efficiency would increase to a degreewhere many more analyses could be performed than were heretoforepossible.

Although the present invention lends itself to a vast range ofapplications, for purposes of clarity this discussion will be couchedlargely in terms of the determination of the ionic strength or specificgravity of urine. Applications to other disciplines will become apparentfrom an understanding of how this invention relates to urinalysis.

The determination of urine specific gravity is of considerable value inthe understanding and clinical management of electrolyte disturbances.Hence, complete urinalysis should, and usually does, include a specificgravity determination. Generally, such a determination would include themeasurement of specific gravity directly with a suitable device, butequally useful is the measurement of some related property, such asosmolality or ionic strength, which can then be referred back tocorresponding specific gravity values.

Specific gravity is a dimensionless term and relates, in the case of asolution, to the ratio of the weight of a certain volume of the solutionto that of an equal volume of water at the same temperature. Forsolutions such as urine, the specific gravity is a function of thenumber, density, ionic charge, and weight of the various species ofdissolved solutes.

Prior art methods for determining specific gravity utilize hydrometers,urinometers, pycnometers, gravimeters and the like. Although these priorart procedures are satisfactorily sensitive in most cases, they allinvolve fragile, bulky instruments which must be constantly cleaned,maintained, and calibrated in order to continuously assure theirreliability. In addition, there are many inconveniences associated withthe mechanics of using these instruments. There may be a difficulty inreading the miniscus. Froth or bubbles on the liquid surface mayinterfere with the reading. There is a tendency for urinometers toadhere to the sides of the vessel containing the liquid sample. In thecase of urine, the sample quantity is frequently inadequate foraccommodating one of the aforementioned devices.

A recent breakthrough in which all of the above disadvantages have beenvirtually eliminated, and which affords rapid osmolality (ergo, specificgravity) determination, is disclosed in U.S. Pat. No. 4,015,462, filedby Greyson et al., on Jan. 8, 1976 and assigned to the present assignee.This patent describes an invention in which a carrier matrix isincorporated with osmotically fragile microcapsules, the walls of whichare composed of a semi-permeable membrane material. Encapsulated insidethe walls is a solution containing a coloring substance. When thecapsules are in contact with a solution having a lower osmolality thanthat within the capsules, an osmotic gradient occurs across the capsulewalls in the direction of the lower osmolality, thereby increasing thehydrostatic pressure within the capsules, thus causing them to swelland, ultimately, to rupture, releasing their colored contents. Theamount of color formed from this phenomenon is a function of thespecific gravity of the solution.

It can be seen from the foregoing that besides the numerous deviceswhich measure specific gravity directly, it is also possible to measurespecific gravity using an indirect means such as the osmolality of asolution. Yet another way of estimating specific gravity withoutmeasuring it directly involves a determination which is proportional tothe ionic strength of a solution. Such an approach is utilized by thepresent invention. It is well known that the specific gravity of anaqueous system is greatly affected by the presence of charged species.Thus, in the case of ionic solutions, it is possible to closelyapproximate the specific gravity of the respective solutions viameasurements proportional to their ionic strengths and referring thosemeasurements to a precalibrated reference system.

The term "ionic strength" refers to the mathematical relationshipbetween the number of different kinds of ionic species in a particularsolution and their respective charges. Thus, ionic strength μ isrepresented mathematically by the formula ##EQU1## in which c is themolal concentration of a particular ionic species and z the absolutevalue of its charge. The sum Σ is taken over all the different kinds ofions in solution.

U.S. Pat. No. 3,449,080 discusses measuring dissolved sodium or chlorideions. This reference is directed to a test device for determining theconcenrations of these ions in body sweat. Briefly, there is disclosedin this patent the use of ion exchange resins together with a pHindicator. Using this device, the presence of sodium or chloride ions issaid to be determined through a color change in the ion exchange resincaused by the pH indicator. Whereas this reference purports to disclosea way of measuring ionic strength, it was found by the present inventorsthat such teachings, as set forth in the examples, were inapplicable tothe measurement of specific gravity.

Both the osmolality approach and the ionic strength approach toindirectly determining specific gravity could conceivably be affectedinsofar as accuracy is concerned by the presence of nonionic species.Accordingly, U.S. Patent Application Ser. No. 716,962, filed Aug. 23,1976, U.S. Pat. No. 4,108,727, is directed to a method for removing thispotential source of inaccuracy, and discloses a device in which thespecific gravity-sensitive system contains an ionizing agent capable ofconverting the nonionic solute to ionized species.

To summarize the present state-of-the-art as it might pertain to thepresent invention, many methods are known for the measurement ofspecific gravity, both direct and indirect. Direct measurement includesutilizing devices which are fragile, bulky and inexpensive, and whichmust be constantly cleaned, maintained and calibrated. Of the indirectmethods, the measurement of the colligative solution property known asosmolality can provide an accurate correlation to specific gravity. Thepresent invention utilizes a different perspective, the relationshipbetween specific gravity and the ionic strength of a solution, andprovides a device, composition and method for taking advantage of thisrelationship. U.S. Pat. No. 3,449,080 describes a method of gauging theconcentration of sodium and/or chloride ions in body sweat. Thisreference utilizes the affinity of weakly acidic or weakly basic ionexchange resins for the unknown ions, and the color changing capacity ofknown pH indicators. None of the prior art known to the presentinventors at the time of filing of the instant application teaches orsuggests the invention presently disclosed and claimed.

SUMMARY OF THE INVENTION

Briefly, the present invention relates to a test means, device, themethod for determining the specific gravity of an aqueous test sample.The test means comprises a weakly acidic or weakly basic polyelectrolytepolymer, which has been at least partially neutralized, and an indicatorsubstance capable of producing a detectable response to ion exchangebetween the polyelectrolyte and the test sample. The device of thepresent invention comprises a carrier matrix incorporated with the testmeans. The method of the present invention comprises contacting a testsample with the device or test means and observing a detectable responsesuch as a change in color, pH or enzyme activity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-8 are graphic portrayals of (a) the responses of threepolyelectrolytes to test samples having verying specific gravities, and(b) the titration of partial neutralization of these polymers. ThusFIGS. 1, 2 and 3 are titration curves for a copolymer of methyl vinylether and maleic anhydride, poly(acrylic acid), and poly(vinylamine)respectively. FIGS. 4, 5 and 6 depict the performances of thesepolyelectrolytes in determining urine specific gravities after varyingdegrees of partial neutralization of the polymer pendant group. FIG. 7shows similar performance of poly(vinylamine) in aqueous salt solutionsof varying concentrations. Finally, FIG. 8 shows the performance of apreferred device.

DETAILED DESCRIPTION OF THE INVENTION

The presently claimed test means comprises, as one ingredient, a weaklyacidic or weakly basic polyelectrolyte. Numerous examples of suchpolymers are known in the art, their common characteristics centeringabout the degree of dissociation of the ionic pendant groups when thepolymer is subjected to an aqueous environment. Most polyelectolytes aresoluble or partially soluble in water, and are readily ionizable,depending on the ionic nature of (a) the aqueous system and (b) theionizable species on the polymer chain.

Thus a polyelectrolyte is branded weakly or strongly ac acidic or basicdepending on its ionic behavior. Generally, a polyelectrolyte whichnearly completely ionizes when contacted with water, such aspoly(vinylsulfuric acid) and poly(styrene sulfonic acid), are consideredstrong polyelectrolytes. Weak polyelectrolytes on the other hand,contain weakly acidic or basic ionizable groups. The charge densityalong the molecular chain of these polymers can be varied by varying thedegree of neutralization. Examples of weakly acidic or weakly basicpolyelectrolytes which find particular applicability to the presentinvention are poly(arcylic acid), poly(maleic acid), maleicacid/methylvinyl ether copolymer, poly(methacrylic acid), styrene-maleicacid copolymer, poly(4-vinylpyridine), and others.

The composition and test means of the present invention includes weaklybasic and weakly acidic polyelectrolytes, but more particularly itincludes those which have been partially neutralized. At least some ofthe functional groups of the polymer, be they weakly acidic (e.g., COOH)or weakly basic, are first partially titrated with a base or acid,respectively, prior to incorporating the polyelectrolyte into the testcomposition. Typically, aqueous solutions of titrant are employed, andbasic titrants include solutions of NaOH, KOH, Na₂ CO₃,poly(ethyleneimine), tris(hydroxymethylamine) methane and others knownto chemists reasonably skilled in the art. Surprisingly, such partialtitration or neutralization has been found to be necessary in order toenable significant differentiation between specific gravity levels intest solutions.

Preferably, the polymer is neutralized to at least about 50%, i.e., atleast about half of the ionizable groups are neutralized. An idealneutralization range, and that presently found most preferred in thepresent invention, is from about 75 to about 95% neutralization, 90%having thus far been found to be optimum in providing the largestseparation in pH change or other detectable response with respect tospecific gravity or ionic strength.

The polyelectrolyte selected for use in the present invention must, asstated supra, be partially neutralized. This is accomplished bytitration of the polymer with suitable acid or base as desired, or byany other means which achieves the desired result of partialneutralization. Thus, FIG. 1 constitutes the titration curve of Gantrez®S-97, a maleic anhydride/methylvinylether copolymer marketed by GeneralAniline and Film Corporation, with sodium hydroxide in aqueous solution.FIG. 2 shows similar data for poly(acrylic acid), and FIG. 3poly(vinylamine).

Another element of the present invention is an indicator means. It cantake on such diverse forms as a pH indicator compound, an enzymaticsystem whose function is responsive to subtle pH changes, a pH meter,and pH-sensitive antigen/antibody systems. Thus, known pH-sensitivechromogenic reagent compounds can be employed, and these can provide achange in or appearance of color, observable by the person performingthe measurement, which is indicative of the ionic strength of specificgravity of the system being tested. If a chromogen is used, a referencecolor system can be established beforehand, so that a quick visualcomparison of the composition and the reference system provides thesought after results. Examples of chromogens suitable for use in thepresent invention are bromothymol blue, alizarin, bromcresol purple,phenol red and neutral red; bromothymol blue having been found to beespecially suitable.

Alternatively, the indicator means can take the form of a pH meter,whereby small changes in pH can be monitored directly, without resortingto visual observation of color change. One particularly suitableapproach is to use the pH meter in conjunction with a surface pHelectrode. The pH meter response can then be observed over various ionicstrength values and a reference system can be established, a particularchange in pH corresponding to a particular test sample ionic strength.

Yet another ramification of the indicator means is a pH-sensitiveenzyme-based system, whereby subtle changes in pH caused by thepolyelectrolyte/ionic strength interaction can trigger the onset ofenzymatic activity, or which can change kinetic reaction parameters suchas the K_(M) for a particular enzymatic reaction. Thus an enzymaticsystem capable of providing a detectable response can be triggered toproduce that response in accordance with the specific gravity or ionicstrength of a test sample. For example, the enzyme chymotrypsin is knownto be sensitive to pH in acting on the substrate p-nitrophenyl acetateto yield p-nitrophenol. Thus the reaction rate dramatically increasesfrom pH 6 to 8, and the appearance of the yellow p-nitrophenol ismarkedly enhanced by pH increases in that range.

Similarly, an antigen-labeled substrate can be employed. The pHdependence of antigen/antibody reactions is well known, and theindicator means of the present invention can include such a labeledsubstrate and the antibody for the label. Change in pH can be measuredby change in substrate available for a corresponding enzymatic reaction.

The present invention includes a device in which a carrier matrix isincorporated with the presently disclosed composition or test means toprovide a tool for obtaining rapid, reliable estimations of solutionspecific gravities. The carrier matrix is usually, but not necessarily,a porous substance such as filter paper. Other art-recognized forms ofcarrier matrix materials are felt, porous ceramic strips, and woven ormatted glass fibers (U.S. Pat. No. 3,846,247). Also suggested are theuse of wood, cloth, sponge material and argillaceous substances (U.S.Pat. No. 3,552,928). All such carrier matrix materials are feasible foruse in the present invention, as are others. It has been found thatfilter paper is especially suitable.

In a preferred embodiment, filter paper is wetted with a solution ofsuspension of a partially neutralized polyelectrolyte in water or othersuitable vehicle easily determinable by routine laboratory experimentsand then dried. The polyelectrolyte-bearing filter paper is subsequentlyincorporated with the desired indicator means. Typically, the paper iswetted with a solution of a pH-sensitive chromogenic indicator (such asbromothymol blue) in methanol or other suitable solvent such as ethanol,N,N-dimethylformamide, dimethylsulfoxide, and subsequently dried.Alternatively, a one-dip method can be used whereby the polyelectrolyteand indicator means are simultaneously present in the initial solutionor suspension.

The dried, reagent-bearing carrier matrix can be mounted on a backingmaterial if desired. The test device, in a preferred embodiment, thuscomprises a filter paper carrier matrix, incorporated with a partiallyneutralized polyelectrolyte and indicator means as described supra, thematrix being affixed to one side to an elongated piece of transparentpolystyrene film. The matrix is secured to one end of the film by anysuitable means, such as double faced adhesive tape (Double Stick®available from 3M Company), the other end of the polystyrene filmserving as a handle. In use, such a device is held by the free end ofthe polystyrene film backing material and the matrix end is immersedinto the test sample (e.g., urine) and quickly removed. Any colorformation or other detectable response is observed after a predeterminedtime and compared with a reference standard corresponding to responsesto known solution ionic strengths or specific gravities.

The particular reference standard employed depends on whether the testmeans is used by itself or incorporated with a carrier matrix, as wellas on the particular indicator means. Thus if the partially neutralizedpolyelectrolyte is added directly to the test sample, and the indicatormeans is a pH meter, a reference standard can be devised by adding astandard weight of polyelectrolyte to a standard volume of a solution ofknown ionic strength. The pH change before and after polyelectrolyteaddition is recorded using the pH meter. This procedure is followed fora series of solutions having varied known ionic strengths. To determinethe ionic strength of an unknown test sample, the same procedure isfollowed by the pH change compared with those for the known solutions.

Where a test device comprising a carrier matrix containing partiallyneutralized polyelectrolyte and a chromogen is employed, a referencestandard can comprise a series of color blocks depicting the colordeveloped by the carrier matrix after a predetermined time in responseto solutions of known ionic strengths. When testing on unknown sample,the carrier matrix of a test device is immersed in the sample, removed,and observed for the appearance of or change in color after thepredetermined time. Any color response is then compared with thereference standard color blocks to ascertain the ionic strength orspecific gravity of the sample.

The following Examples are provided to further teach how to make and usethe present invention. Thus, preferred embodiments are described andanalyzed. The Examples are meant to be illustrative only, and are in noway intended as limiting the scope of the invention described andclaimed herein.

A. THE TEST MEANS Example I--Partial Neutralization of MaleicAnhydride/Methylvinylether Copolymer

This experiment was performed to study the partial neutralization ofpolyelectrolyte (Gantrez S-97 marketed by General Aniline and FilmCorporation), and its effect on a composition for measuring solutionspecific gravity.

A modular automatic titrator was assembled for the titration of variouspolyelectrolytes for study pertinent to the present invention. Thetitrator consisted of an automatic pipetter, Model No. 25000, fromMicromedic Systems, Inc. This instrument is capable of dispensing aconstant volume of titrant per unit time into the polymer solution to betitrated. The rate of addition of titrant (ergo, the rate ofpolyelectrolyte neutralization), was controlled through the selection ofpipette volume, the fraction of pipette volume dispensed, and theconcentration of titrant. Changes in pH during titration were detectedusing a standard pH electrode and an Orion Model 701 digital pH meter.The output of the pH meter was fed into a Hewlett-Packard Model 17500Aten inch strip chart recorder, the scale of which had been calibratedsuch that one inch corresponded to a change of one pH unit. Hence, therecorder provided a continuous monitor of pH changes with respect totime (ergo with respect to volume of titrant added).

This apparatus was used to titrate and observe the effects of partialneutralization on Gantrez S-97, a weakly acidic polyelectrolyte. Asolution of Gantrez was prepared comprising 20 grams of thepolyelectrolyte per liter of deionized water. Three 100 milliliter (ml)aliquots of this solution were placed in 250 ml beakers. One aliquot wasmade 0.1 N and another 1.0 N with NaCl. No salt was added to theremaining aliquot. By titrating each of these polyelectrolyte solutionswith 1.0 N NaOH in a 50 ml pipette at a rate of 9.0 ml. titrant perhour, and recording pH change versus volume of titrant, it was possibleto study the titration characteristics of Gantrez, as well as theeffects of partial neutralization on its ability to differentiatevarying ionic strengths.

The titration data obtained in this experiment is plotted graphically inFIG. 1. Curve A represents the titration of the polyelectrolyte solutionto which no salt was added, curve B represents titration of thepolyelectrolyte solution made 0.1 N in NaCl, and curve C represents thetitration of the polyelectrolyte solution containing NaCl at 1.0 Nconcentration. The clear separation which occurs between curves A, B andC in FIG. 1 is indicative of the effect of ionic strength on theapparent pH of the polymer. Thus, by observing the degrees of separationbetween the titration curves, i.e., of pH values for a given amount oftitrant, one can estimate maximization with respect to determiningdifferent levels of specific gravity. For example, greater separation isobserved in the regions between pH 5 and pH 10 than at other stages ofpolymer neutralization. The curves in FIG. 1 indicate that optimumseparation occurs with a degree of polymer neutralization from about 70%to 95% or more (i.e., addition of about 6.0 to 9.0 ml. titrant). Notonly is this information useful in gauging the effectiveness of thepolymer in aqueous systems, but it also helps towards determiningoptimum neutralization of the polyelectrolyte for incorporation with acarrier matrix as will be seen in Example IV, infra.

The percent neutralization of a given polyelectrolyte can be calculatedfrom titration data such as that presented graphically in FIG. 1 bycurve A (the titration of the polyelectrolyte, here Gantrez S-97, withno added salt). Percent neutralization of the polymer is calculated fora given pH of titrated polymer solution by finding the solution pH onthe vertical axis, extending a horizontal line from the vertical axis tocurve A, and extending a vertical line from that point on curve A to thehorizontal axis (i.e., ml. of 1.0 N NaOH). The volume of titrant(corresponding to the intersection of the vertical line and thehorizontal axis) divided by the titrant volume at the end point oftitration, multiplied by 100 yields a close approximation of the percentof polyelectrolyte neutralization. Titration end point is indicated byvertical linearity of curve A at the far right, and can be expressed interms of the volume of titrant added.

Thus, for Gantrez S-97, the end point shown in FIG. 1 is very close to9.0 (about 8.6) ml of 1.0 N NaOH titrant. Titration of the Gantrezsolution in deionized water to a pH of about 7.5 corresponds to a volumeof about 6.0 ml titrant. Since the end point is about 8.6 ml of titrant,percent neutralization is calculated by ##EQU2##

Example II--Partial Neutralization of Poly(acrylic acid)

This experiment was performed to study the partial neutralization ofpoly(acrylic acid) and the effects of such neutralization on theusefulness of this polyelectrolyte in determining solution specificgravity. The modular automatic titrator, pH meter and electrodedescribed in Example I were employed, as was the procedure.

A solution of the poly(acrylic acid), a weakly acidic polyelectrolyteobtained from Aldrich Chemical Co. (Catalogue No. 19,205-8), wasprepared by dissolving 20 grams of polymer in one liter of deionizedwater. Aliquots of 100 milliliters each of this solution were placed in250 ml beakers. One of the aliquots was made 0.1 N and another 1.0 N inNaCl. No salt was added to the third aliquot. Each of these solutionswas then titrated with 10.0 N NaOH in a 50 ml. pipette at a rate of 3.0ml. titrant per hour. The results are reported in FIG. 2 in which curveA represents the polyelectrolyte solution containing no salt, curve Bthe solution made 0.1 N in NaCl, and curve C the solution made 1.0 N inNaCl.

The data depicted by FIG. 2 illustrates that the greatest separationwith respect to ionic strength (i.e., between curves A, B and C) occursfrom about 50% to about 95% or greater neutralization of the polymer(i.e., addition of about 1.5 to about 3.0 ml. titrant). Thus, forexample, where the polymer has been titrated over a 40 minute period(with 2.0 milliliters of 10 N NaOH), one can see marked separation ofthe resultant pH depending upon the ionic strength of the solution.Curve C which corresponds to 1.0 N NaCl provides a resultant pH value ofabout 5.25, curve B corresponding to 0.1 N NaCl yields a pH value ofabout 5.8, and curve A, which corresponds to zero concentration of NaCl,yields a pH value of about 6.25. Thus, the ionic strength or specificgravity of a particular solution can be approximated by using thesevalues and interpolating between them.

Example III--Partial Neutralization of Poly(vinylamine)

This experiment was performed by study the partial neutralization of aweakly basic polyelectrolyte, poly(vinylamine) obtained from Dynapol,Inc., and the effects of such neutralization on the usefulness of thispolyelectrolyte in determining specific gravity. The modular automatictitrator, pH meter and electrode described in Example I were employed,as well as the procedure.

A solution of poly(vinylamine) in its hydrochloride salt form(completely neutralized) having a molecular weight of about 60,000 wasprepared having a polmer concentration of 20.0 grams per liter ofdeionized water. Three aliquots of 100 milliliters each of this solutionwere placed in 250 ml beakers. One of the aliquots was made 0.5 N andanother 3.0 N in NaCl. No salt was added to the remaining aliquot. Eachof these solutions was then titrated with 1.0 N NaOH using a 50milliliter pipette at a rate of 9.0 ml titrant per hour. The results aredepicted in FIG. 3 in which curve A represents titration of thepolyelectrolyte solution to which no salt was added, curve B thesolution made 0.5 N in NaCl and curve C titration of the solution made3.0 N in NaCl.

The data in FIG. 3 shows that little response occurs with respect toionic strength when the polymer is completely in the amine ornonneutralized form (pH 10, 35 minutes), whereas excellent separationoccurs at lower degrees of titration, i.e., where neutralization of thepolymer is more extensive. Hence, the ability of poly(vinylamine) todifferentiate different ionic strength levels varies inversely with theamount of titrant, such that at the onset of the titration (in excess of95% neutralization) excellent separation is produced, whereas at zeroneutralization (addition of about 5.3 ml. titrant), no separationoccurs.

B. THE TEST DEVICE Example IV--Performance of MaleicAnhydride/Methylvinylether Copolymer in a Carrier Matrix

A test device sensitive to ionic strength or specific gravity wasprepared by incorporating the solution of Gantrez S-97 into filter paperand then drying. Several test devices were prepared in order to studythe performance of the polyelectrolyte at various degrees ofneutralization. Thus, aliquots of the Gantrez S-97 solution wereneutralized to different extents by titration with NaOH. Strips offilter paper obtained from Eaton and Dikeman (No. 204) were respectivelyimmersed in these partially titrated aliquots and subsequently dried.Impregnated dried strips made from each of the aliquots were thenrespectively dipped into urines having different known specificgravities and into deionized water, and the pH thereof was measured. ApH meter having a flat surface electrode obtained from Markson Science,Inc. (No. 1207 BactiMedia combination pH/reference electrode) was usedfor these measurements. The values of ΔpH, i.e., the difference in thepH of identical strips dipped respectively into deionized water andurine of known specific gravity, are tabulated below.

    ______________________________________                                        pH of Poly-                                                                             Δ pH Values Produced by Urines of                             electrolyte                                                                             Indicated Specific Gravities                                        Solution  Sp. Gr.      Sp. Gr. Sp. Gr.                                        Aliquots  1.030        1.015   1.005                                          ______________________________________                                        4.75      0.20         0.29    0.32                                           6.0       1.04         0.91    0.54                                           7.0       1.70         1.44    0.65                                           8.0       2.41         2.06    1.04                                           9.25      3.24         2.29    1.09                                           9.75      3.52         2.66    1.65                                           ______________________________________                                    

The data in the above table has been plotted in FIG. 4, wherein thethree curves represent values of ΔpH produced by urines having theindicated specific gravity values when tested with strips made fromaliquots of different degrees of neutralization. It can be seen fromFIG. 4 that the degree of separation of the curves increases markedly asthe degree of neutralization of the polyelectrolyte, i.e., the pH,increases. Thus, the more partial neutralization of the Gantrezpolyelectrolyte, the greater the ability to differentiate betweenspecific gravity levels in urine.

Example V--Performance of Poly(acrylic acid) in a Carrier Matrix

The polyelectrolyte employed in Example II (20 grams of poly(acrylicacid) per liter of deionized water) was further studied to observe itsbehavior in measuring urine specific gravity when incorporated with acarrier matrix.

Test devices were prepared and tested as in Example IV, except thatpoly(acrylic acid) was substituted for Gantrez S-97. A solution of 20grams of poly(acrylic acid) per liter of deionized water was prepared.Aliquots of this solution were titrated with 10 N sodium hydroxide untilthe pH levels stated in the table below were achieved. Strips of filterpaper obtained from Eaton and Dikeman (No. 204) were respectively dippedinto these aliquots and dried. They were then respectively dipped intourines of different known specific gravity and into deionized water andand pH thereof was measured. The value of ΔpH were determined as inExample IV and are tabulated below. A pH meter having a flat surfaceelectrode obtained from Markson Science, Inc. (No. 1207 BactiMediacombination pH/ reference electrode) was used for these measurements.

    ______________________________________                                        pH of Poly-                                                                             Δ pH Values Produced by Urines                                electrolyte                                                                             of Indicated Specific Gravities                                     Solution  Sp. Gr.      Sp. Gr. Sp. Gr.                                        Aliquots  1.005        1.015   1.030                                          ______________________________________                                        4.0       0.11         0.05    0.00                                           5.0       0.5          0.64    0.70                                           6.0       0.56         0.88    1.09                                           7.0       0.76         1.29    1.68                                           7.5       0.91         1.40    1.98                                           8.0       1.15         1.73    2.29                                           8.25      1.10         1.82    2.10                                           ______________________________________                                    

The data in the above table is plotted in FIG. 5, which, like FIG. 4,shows that the degree of separation of the curves therein increasesmarkedly as the degree of neutralization, i.e., the pH, of thepolyelectrolyte increases.

Example IV--Performance of Poly(vinylamine) in a Carrier Matrix

The polyelectrolyte employed in Example III was further studied toobserve its behavior in measuring various urine specific gravities whenincorporated with a carrier matrix.

Test devices were prepared and tested as in Examples IV and V exceptthat poly(vinylamine) was substituted for Gantrez S-97 and poly(acrylicacid), respectively. A solution was prepared comprising 20 grams ofpoly(vinylamine) (obtained from Dynapol, Inc. 60,000 M.W., see Dawson etal., J.A.C.S. 98, 5996, 1976) per liter of deionized water. Thepolyelectrolyte used was in the hydrochloride form and thus was in thecompletely neutralized state. Aliquots of this solution wererespectively titrated with 1.0 N NaOH to produce the solution pH levelsstated in the table below. Strips of filter paper obtained from Eatonand Dikeman (No. 204) were respectively dipped into these aliquots anddried. They were then respectively dipped into different known specificgravity urines and into deionized water and the pH thereof was measuredusing the flat surface electrode described in Examples IV and V. Thevalues of ΔpH were then determined as in Examples IV and V and aretabulated below.

    ______________________________________                                        pH of Poly-                                                                             Δ pH Values Produced by Urines                                electrolyte                                                                             of Indicated Specific Gravities                                     Solution  Sp. Gr.      Sp. Gr. Sp. Gr.                                        Aliquots  1.005        1.015   1.030                                          ______________________________________                                        2.8       1.05         1.60    1.78                                           3.0       1.06         1.64    1.92                                           3.5       .89          1.11    1.29                                           4.0       .89          1.17    1.21                                           6.0       +.06         -.10    -.33                                           8.0       -0.71        -.99    -1.70                                          10.0      -1.45        -1.47   -2.24                                          11.0      -1.80        -2.50   -2.81                                          12.0      -2.24        -2.57   -3.07                                          ______________________________________                                    

The graph of this data, FIG. 6, portrays useful separation when thepolyelectrolyte is partially neutralized to below about pH 5. Thus thecurve for urine having a specific gravity of 1.005 results in a muchsmaller change in pH than for urine at a specific gravity of 1.030. Theurine having a specific gravity of 1.015 resulted in intermediate ΔpHvalues as expected.

This effect is even more dramatically demonstrated when thepoly(vinylamine) test devices are respectively dipped into aqueous saltsolutions of different ionic strengths and into deionized water and thepH thereof measured to provide ΔpH values. Thus strips prepared as abovewere tested with various concentrations of sodium chloride in deionizedwater. Specifically these salt solution concentrations were 0.5, 1.5 and3.0 N in NaCl. The data obtained in this experiment is tabulated belowand plotted in FIG. 7. Curves A, B and C correspond to salt solutions of3.0, 1.5 and 0.5 N in NaCl, respectively.

    ______________________________________                                        pH of Poly-                                                                             Δ pH Values Produced by Urines                                electrolyte                                                                             of Indicated Specific Gravities                                     Solution  0.5N         1.5N    3.0N                                           Aliquots  NaCl         NaCl    NaCl                                           ______________________________________                                        2.8       .60          .63     .93                                            3.0       1.04         1.37    1.46                                           3.5       .88          .99     1.23                                           4.0       1.00         1.32    1.61                                           6.0       .90          .93     1.03                                           8.0       .64          .64     .68                                            10.0      .61          .64     .60                                            11.0      -.01         -.03    -.10                                           12.0      -.09         -.22    -.29                                           ______________________________________                                    

Referring to FIG. 7, at pH 10 where the polyelectrolyte is essentiallyunprotonated and uncharged, the effect of varying salt concentration isvirtually nonexistent. Partial neutralization of the Polymer, however,effects a steadily increasing divergence of performance in response toionic strength, as evidenced by the increasing difference between therespective plots reflecting widely divergent ΔpH response to differingionic strengths.

Example VII--Test Device Prepared Using MaleicAnhydride/Methylvinylether Copolymer and Bromothymol Blue

The test composition of Example I was employed in a carrier matrixtogether with bromothymol blue, a known pH indicator, to dtudy thecharacteristics of the present invention with respect to visualdetermination of specific gravity.

A solution was prepared containing 20 grams of Gantrez S-97 per liter ofdeionized water. An aliquot of this solution was titrated with NaOHuntil the resultant solution pH was 8.0 as measured with the pH meterand electrode described in Example I. A strip of filter paper (Eaton &Dikeman No. 204) was immersed in the partially titrated (neutralized)aliquot and subsequently dried. The dried polymer-bearing strip was thenimmersed in a methanol solution of bromothymol blue at a concentrationof 1.2 grams per liter. After drying, the filter paper strip was mountedon a clear plastic backing material (Trycite, obtained from Dow ChemicalCo.) using double faced adhesive tape (Double Stick, obtained from 3MCompany). The resultant test devices each comprised a strip of Trycitemeasuring about 3.5 in. by 0.2 in., one end of which bore a square ofthe impregnated filter paper measuring 0.2 in. on a side. The rest ofthe Trycite served as a handle.

The sensitivity of these test devices to specific gravity was studied bytesting with three different specific gravity urine samples and withwater. A device was immersed in the particular test solution and quicklyremoved. After 60 seconds the device was examined in reflectancespectrophotometer which scans and measures the intensity of reflectedlight from the test device over the visible spectral regions every halfa second.

The data obtained at 60 seconds is plotted in FIG. 8 and shows markedseparation enabling easy and accurate specific gravity differentiationbetween water (specific gravity 1.000) and urines at specific gravitylevels of 1.005, 1.015 and 1.030. Visual color differentiation wasequally easy, the device exhibiting a blue color with water, blue-greenwith urine at specific gravity 1.005, green at 1.015 and yellow at1.030.

This example demonstrates the relationship between partialpolyelectrolyte neutralization and specific gravity or ionic strengthdetermination. The Gantrez solution from which the device was made had apH of about 8. Referring to curve A of FIG. 1, this pH corresponds toabout 6.8 ml of titrant. Using the calculation described in Example I,this corresponds to about 79% neutralization. The remarkabledifferentiation between specific gravity levels realized in theforegoing experiment is attributable to this relatively high degree ofpolyelectrolyte neutralization.

What is claimed is:
 1. A test means for determining the ionic strengthor specific gravity of an aqueous test sample, said test meanscomprising a weakly acidic or weakly basic polyelectrolyte polymer, saidpolymer being at least about 50 percent neutralized, and indicator meanscapable of producing a detectable response to ion exchange between saidpolyelectrolyte and said sample.
 2. The test means of claim 1 in whichsaid polymer is a weakly acidic polyelectrolyte.
 3. The test means ofclaim 1 in which said polymer is a weakly basic polyelectrolyte.
 4. Thetest means of claim 1 in which said polyelectrolyte is poly(acrylicacid), poly(maleic acid), maleic acidvinylmethyl ether copolymer,poly(methacrylic acid), styrenemaleic acid copolymer, poly(vinylamine),or poly(4-vinylpyridine).
 5. The test means of any one of claims 1-4 inwhich said polyelectrolyte polymer is about 75 to about 95 percentneutralized.
 6. The test means of any one of claims 1-4 in which saidindicator means is a pH indicator substance.
 7. The test means of anyone of claims 1-4 in which said indicator means is a pH indicatorsubstance, and said polyelectrolyte polymer is about 75 to about 95percent neutralized.
 8. The test means of claim 1 in which saidpolyelectrolyte is a methylvinylether-maleic acid copolymer, and inwhich said indicator means is bromothymol blue.
 9. A test device fordetermining the ionic strength or specific gravity of an aqueous testsample, said device comprising a carrier matrix incorporated with thetest means of any of claims 1-4 or
 8. 10. A test device for determiningthe ionic strength or specific gravity of an aqueous test sample, saiddevice comprising a carrier matrix incorporated with the test means ofclaim
 5. 11. A test device for determining the ionic strength orspecific gravity of an aqueous test sample, said device comprising acarrier matrix incorporated with the test means of claim
 6. 12. A testdevice for determining the ionic strength of specific gravity of anaqueous test sample, said device comprising a carrier matrixincorporated with the test means of claim
 7. 13. A method fordetermining the ionic strength or specific gravity of an aqueous testsample, said method comprising contacting said sample with the testmeans of any of claims 1-4 or 8 and observing a detectable response. 14.A method for determining the ionic strength or specific gravity of anaqueous test sample, said method comprising contacting said sample withthe test means of claim 5 and observing a detectable response.
 15. Amethod for determining the ionic strength or specific gravity of anaqueous test sample, said method comprising contacting said sample withthe test means of claim 6 and observing a detectable response.
 16. Amethod for determining the ionic strength or specific gravity of anaqueous test sample, said method comprising contacting said sample withthe test means of claim 7 and observing a detectable response.
 17. Amethod for determining the ionic strength or specific gravity of anaqueous test sample, said method comprising contacting said sample withthe device of claim 9 and observing a detectable response.
 18. A methodfor determining the ionic strength or specific gravity of an aqueoustest sample, said method comprising contacting said sample with thedevice of claim 10 and observing a detectable response.
 19. A method fordetermining the ionic strength or specific gravity of an aqueous testsample, said method comprising contacting said sample with the device ofclaim 11 and observing a detectable response.
 20. A method ofdetermining the ionic strength or specific gravity of an aqueous testsample, said method comprising contacting said sample with the device ofclaim 12 and observing a detectable response.
 21. A method for preparinga test device for determining the ionic strength or specific gravity ofan aqueous test sample, said method comprising the steps ofneutralizingat least 50 percent of the ionizable groups of a weakly acidic or weaklybasic polyelectolyte polymer, and incorporating said polymer and a pHindicator with a carrier matrix.
 22. The method of claim 21 wherein saidpolyelectrolyte polymer is about 75 to about 95 percent neutralized. 23.The method of claims 21 or 22 wherein said polymer and said pH indicatorare incorporated with said matrix by combination thereof with a suitablesolvent to form an impregnation mixture, and said carrier matrix iscontacted with said mixture and dried.
 24. A method for preparing a testdevice for determining the ionic strength or specific gravity of anaqueous test sample, said method comprising the steps ofpreparing anaqueous solution of a copolymer of maleic acid and methylvinylether,adding an aqueous solution of a base to said copolymer solutionsufficient to neutralize at least 50 percent of the acid groups of saidcopolymer. contacting a carrier matrix with the partly neutralizedpolymer solution thereby incorporating said polymer with said matrix,drying the polymer-incorporated matrix, and incorporating a pH indicatorwith said dried matrix.
 25. The method of claim 24 wherein said pHindicator is bromothymol blue and said indicator is incorporated withsaid dried polymer-incorporated matrix by preparing a solution of saidindicator in methanol, and contacting said polymer-incorporated matrixwith said indicator solution.